Hybrid In-Band-On-Channel (IBOC) DAB systems have been developed that broadcast digital audio signals simultaneously with analog Amplitude Modulation (AM) programs in the AM band. Such systems are also referred to as IBOC-AM systems. In these systems, both the power and bandwidth allocated for digital audio transmission are extremely limited. As a result, these systems are generally unable to add enough redundancy for error control using conventional concatenated coding techniques. However, it is well known that the decoded audio quality can be improved in the presence of transmission errors through the use of an error mitigation and concealment algorithm in the audio decoder. Effective utilization of such an error mitigation and concealment algorithm generally requires error flags that indicate the quality of the channel decoded bit stream. These error flags can be generated using block codes such as Reed Solomon (RS) codes.
FIG. 1 shows a simplified block diagram of a transmitter portion of a conventional IBOC-AM system 100. The system 100 includes an audio coder 102, a block encoder 104, and a modulator 106. An audio signal is applied to the audio encoder 102 and the resulting compressed audio bit stream is applied to a block encoder 104 which may be an RS encoder. The output RS code symbols from the encoder 104 are mapped in the modulator 106 to modulation symbols, e.g., symbols of a Quadrature Amplitude Modulation (QAM) signal set. The resulting modulation symbols are transmitted over an additive noise channel 108 to a receiver (not shown).
Conventional techniques involving mapping of RS-coded symbols directly to QAM symbols in a system such as that shown in FIG. 1 typically use RS codes based on a Galois field GF(2m) if the modulation is 2m-QAM. Thus, each m-bit symbol can map directly into an m-bit QAM symbol. However, this limits the choice of RS codes one can apply and thus may affect the error-correction capability. For example, if 32-QAM modulation is used, a perfectly-matched RS code will be based on GF(25). For a given coding rate of 0.8, an RS code of (30, 24) can be applied to match the resulting 5-bit RS code symbols to the 5-bit QAM symbols. Since longer block codes provide more powerful error protection, it may be desirable to use a (60, 48) RS code based on GF(26) because it can correct twice as many random errors as the (30, 24) code. However, this will result in a mismatch between the 6-bit RS code symbols and the 5-bit QAM symbols.
Multilevel coding is a well-known technique for addressing the above-described mismatch problem. Instead of applying an RS code or other block code to the data bits and then mapping the resulting code symbols into an m-bit modulation symbol as shown in FIG. 1, multilevel coding is a joint coding-modulation technique that applies a different single or concatenated code of appropriate rate to each bit of a given m-bit modulation symbol. Examples of multilevel coding are described in G. J. Pottie and D. P. Taylor, “Multilevel Codes Based on Partitioning,” IEEE Transactions on Information Theory, Vol. 35, No. 1, pp. 87–98, January 1989, H. Imai and S. Hirakawa, “A New Multilevel Coding Method Using Error-Correcting Codes,” IEEE Transactions on Information Theory, Vol. IT-23, No. 3, pp. 371–377, May 1997, and E. Husni and P. Sweeney, “Robust Reed Solomon Coded MPSK Modulation,” Cryptography and Coding, Lecture Notes in Computer Science, Vol. 1355, Springer, pp. 143–154, 1997. The latter reference describes a technique which uses RS codes as component codes for multilevel coding combined with M-ary Phase Shift Keying (MPSK) modulation in place of the above-noted QAM modulation.
A significant problem with existing multilevel coding techniques such as those described in the above-cited references is that these techniques fail to provide optimal performance in IBOC-AM DAB systems and other important bandwidth-limited communication system applications. A need therefore exists in the art for improved multilevel coding techniques.